Update:NASA's Upper Atmosphere Research Satellite fell back to Earth early on Saturday morning. NASA says it has not yet determined the exact point of re-entry, but the vast majority of the satellite's trajectory at the time was over water. However, it also passed over parts of west Africa and northern Canada. There have been no reports of damage or injury.

"This is the largest NASA satellite to come back uncontrolled for quite a while," says Nick Johnson, chief scientist for NASA's Orbital Debris Program Office at the Johnson Space Center in Houston, Texas.

The Upper Atmosphere Research Satellite will come home with a bang (Image: NASA)

Most of the satellite's mass should burn up but the agency has identified 26 parts likely to survive, including a 150-kilogram instrument mount and three 50-kilogram batteries. In total, more than half a tonne of debris should crash on Earth in a debris footprint some 700 kilometres long.

That's well above the risk of 1 in 10,000 that NASA now requires before it launches a satellite, but UARS was designed and launched before NASA drew up those rules.

Johnson points out that no one has ever been seriously injured by space debris, although there are anecdotal reports that a cow was killed by debris from the Skylab space station, which re-entered over Australia in 1979.

Hugh Lewis, an expert in space-debris modelling at the University of Southampton, UK, agrees that the risk is low. "You have to remember that over 20,000 objects larger than 10 centimetres wide have re-entered the atmosphere since the beginning of the space age - and there have been no reported injuries from these events," he says. "At 1 in 3200, the probability of the surviving pieces of UARS causing injury are remote"

NASA has not yet said where UARS will hit, other than somewhere between 57 degrees north and 57 degrees south. That area spans northern Canada through to the southern tip of South America and most of Europe and Asia and so encompasses most of the world's 7 billion population, along with a huge swathe of ocean.

Any wannabe space-junk collectors, be warned. On its website, NASA says: "If you find something you think may be a piece of UARS, do not touch it. Contact a local law enforcement official for assistance."

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